An optical carrier (fiber optic) transmission medium such as the optical carrier medium operating within the synchronous optical network (SONET) standard adopted by the American National Standards Institute, Inc. (ANSI) as set forth in their Standard Number TI.105-1988 entitled "Digital Hierarchy Optical Interface Rates and Formats Specification" is increasingly being accepted for the communication of many forms of digital signals. According to the SONET standard, an optical carrier level (such as OC-1, OC-3, OC-12, OC-48) is the signal that results from an optical conversion of an electrical synchronous transport signal (STS) operating at the same level. The STS-1 (STS level one) is defined as the basic SONET building block signal with a transfer rate of 51.840 megabits per second (Mbit/s).
An optical carrier is typically used to carry high speed optical signals, e.g., an OC-3 channel operating at 155.52 Mbit/s, and typically over long distances. At various locations along the optical carrier, interfaces may be provided for interfacing electrical (digital) signals lines, e.g., DS-1, DS-3, with the optical carrier. At the interface between the optical signal line and the digital signal line, a known type of add/drop multiplexer (ADM) may be provided which allows multiple functions such as the manipulation of circuits at the DS-1 and DS-3 levels through a variety of low speed interfaces. The low speed and lower level signals can ultimately be used to connect with system end users.
In certain applications, it may be desirable to provide the optical carrier with an interface with an optical extension (intermediate optical fiber signal bus) before breaking down the signal into the lower level electronic signals. Such an optical extension could be used to provide a signal transmission path for optical signals from the interface with the OC-N/STS-N optical signal bus and a remote location. This would allow the continued transmission benefits associated with SONET signal transmission to the remote location. However, within such an interface, there is a need to provide a transport frame format and line rate for transmitting optical signals on such an optical extension. One possible method would be to use the digital signal standard, or DS-X standard, for the transmission of such signals. However, such a standard does not provide the extensive operations, administration and maintenance (OAM) capabilities of the SONET standard.